1. Field
Dry granular fertilizer blending operations, dry animal feed blending, dry granular chemical blending and granular blending operations, in general, all have a common problem, namely, air contamination from finely dispersed particles of the materials being handled. Such finely dispersed dusts can be a source of inconvenience and irritation to the human operators of the facility, a possible combustion or explosion hazard in the case of some materials, and, in any event, a substantial loss of product if the dispersed materials are discarded as a waste material as is done with conventional spent filter media.
2. Prior Art
The problems associated with fine dusts suspended in the air in dry fertilizer are well known. Three general types of dust collectors have been in use: medium pressure drop wet collectors, cloth shakers (continuous) and venturis (high pressure wet collectors). See Bonn, David E., Proc. Ann Meet., Fert. Ind. Round Table, 15th, Washington, D.C., pages 44-7 (Eng.), 1965.
U.S. Pat. No. 3,861,889 issued Jan. 21, 1975, describes a dust filter system for use in fertilizer plants in which a gas stream is sprayed with a solvent and passed through a solvent-wetted layer of foamed material (ether type polyurethane foam resin) having a noncellular porosity of over 90%. In such a system the trapped dust particles represent a loss, and the added solvent adds a cost to the dust removal process. In other prior commercial systems, the product was sprayed with an oil to keep dust down. However, spraying is becoming too expensive because oil costs have skyrocketed.
Baker, et al have suggested that kiln dust from cement factories can be used to replace hydrated lime in a vacuum filter process for treating sewage sludge. See Baker, Dale E. et al, Compost Sci. 16(4):28-30 (Eng.) 1975. It was suggested here that the filter cake or effluent from the vacuum filtration process could then be used as sources of nutrients for crop production.
None of the above references disclose the use of a granular filter medium consisting essentially of the same material which is suspended in the gaseous atmosphere to collect the suspended particles and purify the atmosphere. This granular material, when laden with trapped particles can then be added to the product stream, resulting in substantial product savings.